Nitride-based semiconductor laser device

ABSTRACT

A nitride-based semiconductor laser device capable of elongating the life thereof is obtained. This nitride-based semiconductor laser device comprises a first cladding layer consisting of a first conductivity type nitride-based semiconductor, an emission layer, formed on the first cladding layer, consisting of a nitride-based semiconductor and a second cladding layer, formed on the emission layer, consisting of a second conductivity type nitride-based semiconductor, while the emission layer includes an active layer emitting light, a light guiding layer for confining light and a carrier blocking layer, arranged between the active layer and the light guiding layer, having a larger band gap than the light guiding layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitride-based semiconductor laserdevice, and more particularly, it relates to a nitride-basedsemiconductor laser device formed by successively crystal-growing anactive layer, a cladding layer and the like.

2. Description of the Background Art

A nitride-based semiconductor laser device expected as the light sourcefor an advanced large capacity optical disk has recently been activelydeveloped. In general, a semiconductor laser device has importantcharacteristics such as a threshold current and an operating current.The semiconductor laser device starts lasing with the threshold current,and the operating current is reduced as the threshold current isreduced. Therefore, the threshold current is preferably minimized.

Reduction of the threshold current is generally attempted also in thenitride-based semiconductor laser device. When the threshold current isreduced, the operating current necessary for driving the nitride-basedsemiconductor laser device can also be reduced and hence the device canbe prevented from deterioration resulting from rise of the internaltemperature following increase of the operating current. Thus, it isimportant to reduce the threshold current also for elongating the lifeof the nitride-based semiconductor laser device.

In order to reduce the threshold current, light spreading from anemission layer to a cladding layer must generally be reduced forefficiently confining light in the emission layer for the followingreason: When the difference between the refractive indices of theemission layer and the cladding layer is reduced for increasing lightspreading in a conventional nitride-based semiconductor laser device,the difference between the band gaps of these layers is so reduced as toincrease the number of carriers (electrons and holes) overflowing fromthe emission layer into the cladding layer. If the number of theoverflowing carriers is increased, it is difficult to emit light andhence the threshold current as well as the operating current aredisadvantageously increased. In general, therefore, the differencebetween the refractive index of the emission layer and that of thecladding layer is increased to increase the difference between the bandgaps thereof in order to inhibit the carriers from overflowing andreduce the threshold current. When the difference between the refractiveindices of the emission layer and the cladding layer is increased, lightspreading from the emission layer to the cladding layer is reduced andhence light is efficiently confined in the emission layer. Consequently,optical density is increased in the emission layer to increase thevertical beam divergence angle.

Thus, the conventional nitride-based semiconductor laser device isprepared to increase the vertical beam divergence angle to about 300.For example, Japanese Journal of Applied Physics, Volume 39 (2000),L647-650 discloses such a nitride-based semiconductor laser device. Thenitride-based semiconductor laser device disclosed in this literaturehas a vertical beam divergence angle of 29.9°.

However, the aforementioned conventional nitride-based semiconductorlaser device having a large vertical beam divergence angle has thefollowing problem: The nitride-based semiconductor laser device includesa larger number of crystal defects such as dislocations than an AlGaAs-or AlGaInP-based semiconductor laser device emitting infrared or redlight. Further, the nitride-based semiconductor laser device emitspurple or ultraviolet light having a short wavelength, and suchshort-wavelength light has large energy in lasing. The aforementionedconventional nitride-based semiconductor laser device having a largevertical beam divergence angle formed to reduce light spreadingremarkably confines light in the emission layer. Therefore, the opticaldensity is increased in the device to easily cause light absorptionresulting from crystal defects such as dislocations. Such lightabsorption results in consumption of excess energy, to disadvantageouslyincrease the operating current. When the operating current is increased,the internal temperature of the device rises to reduce the band gap,leading to large light absorption resulting from crystal defects.Consequently, the device is so abruptly deteriorated that it isdifficult to elongate the life of the aforementioned nitride-basedsemiconductor laser device having a large vertical beam divergenceangle.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a nitride-basedsemiconductor laser device capable of elongating the life thereof.

Another object of the present invention is to enable the aforementionednitride-based semiconductor laser device to increase light spreadingfrom an emission layer to a cladding layer.

In order to attain the aforementioned objects, a nitride-basedsemiconductor laser device according to a first aspect of the presentinvention comprises a first cladding layer consisting of a firstconductivity type nitride-based semiconductor, an emission layer, formedon the first cladding layer, consisting of a nitride-basedsemiconductor, and a second cladding layer, formed on the emissionlayer, consisting of a second conductivity type nitride-basedsemiconductor, while the emission layer includes an active layeremitting light, a light guiding layer for confining light and a carrierblocking layer, arranged between the active layer and the light guidinglayer, having a larger band gap than the light guiding layer.

The nitride-based semiconductor laser device according to the firstaspect, provided with the carrier blocking layer having a larger bandgap than the light guiding layer between the active layer and the lightguiding layer as hereinabove described, can inhibit carriers (electronsand holes) from overflowing from the emission layer into the claddinglayers through the carrier blocking layer also when the differencebetween the refractive indices of the emission layer and the claddinglayers is reduced thereby increasing light spreading. Thus, a thresholdcurrent and an operating current can be inhibited from increaseresulting from difficulty in light emission followed by overflow of thecarriers. Consequently, the nitride-based semiconductor laser device canbe inhibited from deterioration resulting from rise of the internaltemperature following increase of the operating current, whereby thelife of the device can be elongated.

In the aforementioned nitride-based semiconductor laser device accordingto the first aspect, the light guiding layers are preferably formed onthe upper and lower surfaces of the active layer respectively, and thecarrier blocking layers are preferably arranged both between the activelayer and the light guiding layer formed on the upper surface of theactive layer and between the active layer and the light guiding layerformed on the lower surface of the active layer. According to thisstructure, the carrier blocking layers can inhibit carriers (electronsand holes) from overflowing from the emission layer into the first andsecond cladding layers.

The aforementioned nitride-based semiconductor laser device according tothe first aspect preferably further comprises a light spreading layer,arranged at least either between the emission layer and the firstcladding layer or between the emission layer and the second claddinglayer, having a smaller refractive index and a larger band gap than theadjacent first or second cladding layer. According to this structure,the light spreading layer can increase light spreading from the emissionlayer to the first or second cladding layer due to the refractive indexsmaller than that of the first or second cladding layer and stronglyconfine carriers in the emission layer due to the band gap larger thanthat of the first or second cladding layer. Thus, light spreading can beso increased as to reduce optical density in the device. Consequently,light absorption resulting from crystal defects can be so reduced as tosuppress increase of the operating current resulting from lightabsorption. Further, the carriers can be strongly confined in theemission layer as hereinabove described, whereby the carriers (electronsand holes) can be further inhibited from overflowing from the emissionlayer into the cladding layers. Therefore, the threshold current and theoperating current can be further inhibited from increase followed byoverflow of the carriers. Thus, the operating current can be furtherinhibited from increase, whereby the device can be further inhibitedfrom deterioration resulting from rise of the internal temperaturefollowing increase of the operating current, so that the life of thedevice can be further elongated.

In this case, the light spreading layers are preferably arranged bothbetween the emission layer and the first cladding layer and between theemission layer and the second cladding layer. According to thisstructure, light spreading from the emission layer to both of the firstand second cladding layers can be increased while the carriers can bemore strongly confined in the emission layer.

In the aforementioned nitride-based semiconductor laser device includingthe carrier blocking layer and the light spreading layer, at leasteither the carrier blocking layer or the light spreading layerpreferably contains one or two elements selected from a group consistingof B, Al, In, Ga and Tl. According to this structure, the carrierblocking layer having a larger band gap than the light guiding layer andthe light spreading layer having a smaller refractive index and a largerband gap than the first or second cladding layer can be easily formed.

In the aforementioned nitride-based semiconductor laser device accordingto the first aspect, at least either the first cladding layer or thesecond cladding layer preferably consists of a nitride containing Al, Gaand In. According to this structure, the difference between the bandgaps of the emission layer and the cladding layers can be kept largealso when the difference between the refractive indices of the emissionlayer and the cladding layers is reduced, whereby the carriers can beeasily strongly confined in the emission layer also when light spreadingfrom the emission layer to the cladding layers is increased.

In the aforementioned nitride-based semiconductor laser device accordingto the first aspect, the carrier blocking layer may consist of a nitridecontaining Al, Ga and In. Further, the nitride-based semiconductor laserdevice may further comprise a light spreading layer, arranged at leasteither between the emission layer and the first cladding layer orbetween the emission layer and the second cladding layer, having asmaller refractive index and a larger band gap than the adjacent firstor second cladding layer, and at least either the carrier blocking layeror the light spreading layer may consist of a nitride containing Al, Gaand In.

At least either the first cladding layer or the second cladding layerpreferably consists of a nitride, containing Al, Ga and In, having alattice constant substantially identical to that of GaN. According tothis structure, formation of crystal defects resulting from differencebetween lattice constants of either cladding layer and an underlayer canbe remarkably suppressed when the underlayer consists of GaN.

In the aforementioned nitride-based semiconductor laser device accordingto the first aspect, the emission layer preferably includes an activelayer consisting of either a single quantum well structure or a multiplequantum well structure. According to this structure, optical confinementcan be easily controlled.

In the aforementioned nitride-based semiconductor laser device accordingto the first aspect, the first cladding layer consisting of the firstconductivity type nitride-based semiconductor is preferably formed on afirst conductivity type nitride-based semiconductor substrate. Accordingto this structure, the first cladding layer can be formed with a smallnumber of crystal defects.

In the aforementioned nitride-based semiconductor laser device accordingto the first aspect, a vertical beam divergence angle is preferably notmore than 20°. According to this structure, light spreading from theemission layer to the first and second cladding layers is so increasedthat optical density in the device can be reduced. Thus, lightabsorption resulting from crystal defects can be so reduced as tosuppress increase of the operating current. Consequently, the device canbe inhibited from deterioration resulting from rise of the internaltemperature following increase of the operating current, whereby thelife of the device can be elongated.

A nitride-based semiconductor laser device according to a second aspectof the present invention comprises a first cladding layer consisting ofa first conductivity type nitride-based semiconductor, an emissionlayer, formed on the first cladding layer, consisting of a nitride-basedsemiconductor, and a second cladding layer, formed on the emissionlayer, consisting of a second conductivity type nitride-basedsemiconductor, while the emission layer includes an active layeremitting light and a light guiding layer formed on at least either theupper surface or the lower surface of the active layer for confininglight, and the nitride-based semiconductor laser device furthercomprises a light spreading layer, arranged between the light guidinglayer and either the first or second cladding layer being formed on thesame side of the light guiding layer, having a smaller refractive indexand a larger band gap than the either first or second cladding layerbeing formed on the same side of the light guiding layer.

In the nitride-based semiconductor laser device according to the secondaspect, as hereinabove described, the emission layer includes the activelayer and the light guiding layer, while the light spreading layerhaving a smaller refractive index and a larger band gap than theadjacent first or second cladding layer is provided between the lightguiding layer and the first cladding layer or between the light guidinglayer and the second cladding layer to be capable of increasing lightspreading from the emission layer to the first or second cladding layerdue to the refractive index smaller than that of the first or secondcladding layer and strongly confining carriers in the emission layer dueto the band gap larger than that of the first or second cladding layer.Thus, light spreading can be so increased that optical density can bereduced in the device. Therefore, light absorption resulting fromcrystal defects can be so reduced as to suppress increase of theoperating current resulting from light absorption. Further, the carrierscan be more strongly confined in the emission layer as hereinabovedescribed, whereby the carriers (electrons and holes) can be moreinhibited from overflowing from the emission layer into the claddinglayer. Therefore, the threshold current and the operating current can bemore inhibited from increase followed by overflow of the carriers. Thus,the operating current can be more inhibited from increase, whereby thedevice can be more inhibited from deterioration resulting from rise ofthe internal temperature following increase of the operating current.Consequently, the life of the device can be more elongated.

An impurity level is formed on the interface between the light guidinglayer and the light spreading layer. Light absorption may result fromthis impurity level to increase the operating current and the thresholdcurrent. In the nitride-based semiconductor laser device according tothe second aspect, the light spreading layer is separated from theactive layer by the thickness of the light guiding layer, wherebyoptical density can be reduced on the position of the interface betweenthe light guiding layer and the light spreading layer. Consequently,light absorption resulting from the impurity level on the aforementionedinterface can be so suppressed as to further inhibit the operatingcurrent and the threshold current from increase.

The aforementioned nitride-based semiconductor laser device according tothe second aspect preferably further includes light guiding layers forconfining light on both of the upper and lower surfaces of the activelayer. According to this structure, the light guiding layers positionedon both sides of the active layer can easily adjust optical confinementin the emission layer.

The aforementioned nitride-based semiconductor laser device according tothe second aspect preferably further comprises another light spreadinglayer arranged between the light active layer and either the second orfirst cladding layer being formed on the opposite side of the lightguiding layer, having a smaller refractive index and a larger band gapthan the either second or first cladding layer being formed on theopposite side of the light guiding layer. According to this structure,the two light spreading layers can increase light spreading from theemission layer to the first and second cladding layers due to therefractive indices smaller than those of the first and second claddinglayers and further strongly confine carriers in the emission layer dueto the band gaps larger than those of the first and second claddinglayers.

The aforementioned nitride-based semiconductor laser device according tothe second aspect preferably further comprises a carrier blocking layerhaving a larger band gap than the light guiding layer between the activelayer and the light guiding layer. According to this structure, thecarrier blocking layer can inhibit carriers (electrons and holes) fromoverflowing from the emission layer into the cladding layer(s) also whenthe difference between the refractive indices of the emission layer andthe cladding layer(s) is reduced thereby increasing light spreading.Thus, the threshold current and the operating current can be furtherinhibited from increase resulting from difficulty in light emissionfollowed by overflow of the carriers.

In the aforementioned nitride-based semiconductor laser device includingthe carrier blocking layer and the light spreading layer, at leasteither the carrier blocking layer or the light spreading layerpreferably contains one or two elements selected from a group consistingof B, Al, In, Ga and Tl. According to this structure, the carrierblocking layer having a larger band gap than the light guiding layer andthe light spreading layer having a smaller refractive index and a largerband gap than the first or second cladding layer can be easily formed.

In the aforementioned nitride-based semiconductor laser device accordingto the second aspect, at least either the first cladding layer or thesecond cladding layer preferably consists of a nitride containing Al, Gaand In. According to this structure, the difference between the bandgaps of the emission layer and the cladding layers can be kept largealso when the difference between the refractive indices of the emissionlayer and the cladding layers is reduced, whereby the carriers can beeasily strongly confined in the emission layer also when light spreadingfrom the emission layer to the cladding layers is increased.

In the aforementioned nitride-based semiconductor laser device accordingto the second aspect, the light spreading layer may consist of a nitridecontaining Al, Ga and In.

At least either the first cladding layer or the second cladding layerpreferably consists of a nitride, containing Al, Ga and In, having alattice constant substantially identical to that of GaN. According tothis structure, formation of crystal defects resulting from differencebetween lattice constants of either cladding layer and an underlayer canbe remarkably suppressed when the underlayer consists of GaN.

In the aforementioned nitride-based semiconductor laser device accordingto the second aspect, the emission layer preferably includes an activelayer consisting of either a single quantum well structure or a multiplequantum well structure. According to this structure, optical confinementcan be easily controlled.

In the aforementioned nitride-based semiconductor laser device accordingto the second aspect, the first cladding layer consisting of the firstconductivity type nitride-based semiconductor is preferably formed on afirst conductivity type nitride-based semiconductor substrate. Accordingto this structure, the first cladding layer can be formed with a smallnumber of crystal defects.

In the aforementioned nitride-based semiconductor laser device accordingto the second aspect, a vertical beam divergence angle is preferably notmore than 20°. According to this structure, light spreading from theemission layer to the first and second cladding layers is so increasedthat optical density in the device can be reduced. Thus, lightabsorption resulting from crystal defects can be so reduced as tosuppress increase of the operating current. Consequently, the device canbe inhibited from deterioration resulting from rise of the internaltemperature following increase of the operating current, whereby thelife of the device can be elongated.

A nitride-based semiconductor laser device according to a third aspectof the present invention comprises a first cladding layer consisting ofa first conductivity type nitride-based semiconductor, an emissionlayer, formed on the first cladding layer, consisting of a nitride-basedsemiconductor, and a second cladding layer, formed on the emissionlayer, consisting of a second conductivity type nitride-basedsemiconductor, and the nitride-based semiconductor laser device reducesthe degree of optical confinement in the emission layer thereby reducinga vertical beam divergence angle to not more than 20°.

The nitride-based semiconductor laser device according to the thirdaspect reduces the degree of optical confinement in the emission layerthereby setting the vertical beam divergence angle to not more than 20°as hereinabove described, whereby light spreading from the emissionlayer into the first and second cladding layers is so increased thatoptical density in the device can be reduced. Thus, light absorptionresulting from crystal defects can be so reduced as to suppress increaseof the operating current. Consequently, the device can be inhibited fromdeterioration resulting from rise of the internal temperature followingincrease of the operating current, so that the life of the device can beelongated.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the structure of a nitride-basedsemiconductor laser device according to a first embodiment of thepresent invention;

FIG. 2 is a sectional view showing the nitride-based semiconductor laserdevice according to the first embodiment of the present invention;

FIG. 3 is a sectional view showing an emission layer of thenitride-based semiconductor laser device according to the firstembodiment shown in FIGS. 1 and 2 in detail;

FIGS. 4 to 7 are sectional views for illustrating a method offabricating the nitride-based semiconductor laser device according tothe first embodiment shown in FIGS. 1 to 3;

FIG. 8 is a sectional view showing a nitride-based semiconductor laserdevice according to a second embodiment of the present invention;

FIG. 9 is a sectional view showing an emission layer of a nitride-basedsemiconductor laser device according to a third embodiment of thepresent invention in detail;

FIG. 10 is a sectional view showing a nitride-based semiconductor laserdevice according to a fourth embodiment of the present invention;

FIGS. 11 to 14 are sectional views for illustrating a method offabricating the nitride-based semiconductor laser device according tothe fourth embodiment shown in FIG. 10;

FIG. 15 is a sectional view showing a nitride-based semiconductor laserdevice according to a fifth embodiment of the present invention;

FIG. 16 is a sectional view showing an emission layer of thenitride-based semiconductor laser device according to the fifthembodiment shown in FIG. 15 in detail;

FIG. 17 is a graph showing the relation between band gaps and refractiveindices of ternary and quaternary mixed crystal materials in relationthe fifth embodiment of the present invention obtained by calculation;

FIG. 18 is a sectional view showing a nitride-based semiconductor laserdevice according to a sixth embodiment of the present invention;

FIG. 19 is a sectional view showing an emission layer of thenitride-based semiconductor laser device according to the sixthembodiment shown in FIG. 18 in detail;

FIG. 20 is a sectional view showing a nitride-based semiconductor laserdevice according to a seventh embodiment of the present invention;

FIG. 21 is a sectional view showing an emission layer of thenitride-based semiconductor laser device according to the seventhembodiment shown in FIG. 20 in detail; and

FIG. 22 is a graph showing the relation between band gaps and refractiveindices of ternary and quaternary mixed crystal materials,lattice-matching with GaN, in relation to the seventh embodiment of thepresent invention obtained by calculation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are now described with reference tothe drawings.

First Embodiment

The structure of a nitride-based semiconductor laser device according toa first embodiment of the present invention is described with referenceto FIGS. 1 to 3. In the nitride-based semiconductor laser deviceaccording to the first embodiment, an n-type contact layer 2 of n-typeGaN having a thickness of about 4 μm is formed on a sapphire substrate1, as shown in FIGS. 1 and 2. An n-type cladding layer 3 of n-typeAl_(0.03)Ga_(0.97)N having a thickness of about 1 μm is formed on then-type contact layer 2. According to the first embodiment, the Alcomposition of the n-type cladding layer 3 is reduced for reducing thedifference between the refractive index thereof and that of an emissionlayer 5 described later, thereby increasing light spreading from theemission layer 5 into the n-type cladding layer 3.

According to the first embodiment, an n-type light spreading layer 4 ofn-type Al_(0.15)Ga_(0.85)N having a thickness of about 20 nm is formedon the n-type cladding layer 3. This n-type light spreading layer 4 hasa smaller refractive index and a larger band gap than the n-typecladding layer 3. The n-type cladding layer 3 is an example of the“first cladding layer” in the present invention, and the n-type lightspreading layer 4 is an example of the “light spreading layer” in thepresent invention.

The emission layer 5 consisting of a multilevel film structure is formedon the n-type light spreading layer 4. As shown in FIG. 3, the emissionlayer 5 consisting of a multilevel film structure has a multiple quantumwell (MQW) active layer formed by alternately stacking three quantumwell layers 51 of In_(x)Ga_(1-x)N each having a thickness of about 4 nmand four quantum barrier layers 52 of In_(y)Ga_(1-y)N each having athickness of about 20 nm, where x>y and x=0.13 and y=0.05 in the firstembodiment.

According to the first embodiment, an n-type carrier blocking layer 53of n-type Al_(0.15)Ga_(0.85)N having a thickness of about 20 nm isformed under the lower surface of the MQW active layer while an n-typelight guiding layer 54 of n-type GaN having a thickness of about 100 nmis formed under the n-type carrier blocking layer 53. The n-type carrierblocking layer 53 is formed to have a larger band gap than the n-typelight guiding layer 54. The n-type carrier blocking layer 53 hasfunctions of inhibiting carriers from overflowing and spreading light. Ap-type carrier blocking layer 55 of p-type Al_(0.15)Ga_(0.85)N having athickness of about 20 nm is formed on the upper surface of the MQWactive layer while a p-type light guiding layer 56 of p-type GaN havinga thickness of about 100 nm is formed on the p-type carrier blockinglayer 55. The p-type carrier blocking layer 55 is formed to have alarger band gap than the p-type light guiding layer 56. The p-typecarrier blocking layer 55 has functions of inhibiting carriers fromoverflowing and spreading light.

The MQW active layer is an example of the “active layer” in the presentinvention, and the n- and p-type carrier blocking layers 53 and 55 areexamples of the “carrier blocking layer” in the present invention. Then- and p-type light guiding layers 54 and 56 are examples of the “lightguiding layer” in the present invention.

According to the first embodiment, further, a p-type light spreadinglayer 6 of p-type Al_(0.15)Ga_(0.85)N having a thickness of about 20 nmis formed on the emission layer 5. A p-type cladding layer 7 ofAl_(0.03)Ga_(0.97)N having a projecting portion is formed on the p-typelight spreading layer 6. The p-type light spreading layer 6 has asmaller refractive index and a larger band gap than the p-type claddinglayer 7. According to the first embodiment, the Al composition of thep-type cladding layer 7 is reduced for reducing the difference betweenthe refractive indices of the emission layer 5 and the p-type claddinglayer 7. Thus, light spreading from the emission layer 5 to the p-typecladding layer 7 is increased. The p-type cladding layer 7 is an exampleof the “second cladding layer” in the present invention, and the p-typelight spreading layer 6 is an example of the “light spreading layer”inthe present invention.

The thickness of the projecting portion of the p-type cladding layer 7is about 0.3 μm, while that of the remaining region is about 0.1 μm. Ap-type contact layer 8 of GaN having a thickness of about 0.07 μm isformed on the upper surface of the projecting portion of the p-typecladding layer 7. The p-type contact layer 8 and the projecting portionof the p-type cladding layer 7 form a ridge portion 9.

The regions from the p-type cladding layer 7 to the n-type contact layer2 are partially removed. A current blocking layer 10 of SiO₂ having athickness of about 0.2 μm is formed on the removed and exposed part ofthe n-type contact layer 2, the exposed side surfaces of the n-typecladding layer 3, the n-type light spreading layer 4, the emission layer5, the p-type light spreading layer 6, the p-type cladding layer 7 andthe p-type contact layer 8, the upper surface of the region of thep-type cladding layer 7 excluding the ridge portion 9 and the uppersurface of the ridge portion 9 in the vicinity of a cavity end surface.

A p-side ohmic electrode 11 consisting of a lower Pt layer having athickness of about 1 nm and an upper Pd layer having a thickness ofabout 3 nm is formed on the p-type contact layer 8. Further, a p-sidepad electrode 12 consisting of a lower Ni layer having a thickness ofabout 0.1 μm and an upper Au layer having a thickness of about 3 μm isformed on the p-side ohmic electrode 11 and the current blocking layer10.

In addition, an n-side ohmic electrode 13 consisting of a lower Ti layerhaving a thickness of about 10 nm and an upper Al layer having athickness of about 0.1 μm is formed on a part of the exposed portion ofthe n-type contact layer 2 formed with no current blocking layer 10. Ann-side pad electrode 14 consisting of a lower Ni layer having athickness of about 0.1 μm and an upper Au layer having a thickness ofabout 3 μm is formed on the n-side ohmic electrode 13.

According to the first embodiment, as hereinabove described, the n- andp-type carrier blocking layers 53 and 55 having larger band gaps thanthe n- and p-type light guiding layers 54 and 56 are provided betweenthe MQW active layer and the n-type light guiding layer 54 and betweenthe MQW active layer and the p-type light guiding layer 56 respectivelyin the emission layer 5. Also when light spreading is increased,therefore, the n- and p-type carrier blocking layers 53 and 55 caninhibit carriers from overflowing from the emission layer 5 into the n-and p-type cladding layers 3 and 7. Thus, a threshold current and anoperating current can be inhibited from increase resulting fromdifficulty in light emission followed by overflow of the carriers.Consequently, the nitride-based semiconductor laser device can beinhibited from deterioration resulting from rise of the internaltemperature following increase of the operating current, whereby thelife of the device can be elongated.

According to the first embodiment, further, the n- and p-type lightspreading layers 4 and 6 having smaller refractive indices and largerband gaps than the n- and p-type cladding layers 3 and 7 are providedbetween the emission layer 5 and the n- and p-type cladding layers 3 and7 respectively, so that the n- and p-type light spreading layers 4 and 6can increase light spreading from the emission layer 5 to the n- andp-type cladding layers 3 and 7 due to the refractive indices smallerthan those of the n- and p-type cladding layers 3 and 7 and stronglyconfine carriers in the emission layer 5 due to the band gaps largerthan those of the n- and p-type cladding layers 3 and 7. Thus, lightspreading can be so increased as to reduce optical density in thedevice. Therefore, light absorption resulting from crystal defects canbe so reduced as to suppress increase of the operating current resultingfrom light absorption. Further, the carriers (electrons and holes),which can be strongly confined in the emission layer 5 as hereinabovedescribed, can be further inhibited from overflowing from the emissionlayer 5 into the n- and p-type cladding layers 3 and 7. Thus, thethreshold current and the operating current can be further inhibitedfrom increase followed by overflow of the carriers. Thus, the operatingcurrent can be further inhibited from increase, whereby the device canbe further inhibited from deterioration resulting from rise of theinternal temperature following increase of the operating current, andthe life of the device can consequently be further elongated.

In the nitride-based semiconductor laser device according to the firstembodiment, further, light spreading can be increased as hereinabovedescribed, whereby the vertical beam divergence angle in lasing can bereduced to about 16°. Thus, the vertical beam divergence angle can beremarkably reduced as compared with a conventional nitride-basedsemiconductor laser device prepared to have a large vertical beamdivergence angle of about 30°.

A method of fabricating the nitride-based semiconductor laser deviceaccording to the first embodiment is now described with reference toFIGS. 4 to 7.

First, the n-type contact layer 2 of n-type GaN having the thickness ofabout 4 μm, the n-type cladding layer 3 of n-type Al_(0.03)Ga_(0.97)Nhaving the thickness of about 1 μm, the n-type light spreading layer 4of n-type Al_(0.15)Ga_(0.85)N having the thickness of about 20 nm andthe emission layer 5 consisting of a multilevel film structure aresuccessively formed on the sapphire substrate 1 by MOCVD (metal organicchemical vapor deposition), as shown in FIG. 4.

In order to form the emission layer 5 consisting of a multilevel filmstructure, the n-type light guiding layer 54 of n-type GaN having thethickness of about 100 nm and the n-type carrier blocking layer 53 ofn-type Al_(0.15)Ga_(0.85)N having the thickness of about 20 nm aresuccessively formed on the n-type light spreading layer 4, as shown inFIG. 3. Then, the four quantum barrier layers 52 of In_(y)Ga_(1-y)N eachhaving the thickness of about 20 nm and the three quantum well layers 51of In_(x)Ga_(1-x)N each having the thickness of about 4 nm arealternately successively formed on the n-type carrier blocking layer 53,thereby forming the MQW active layer. Then, the p-type carrier blockinglayer 55 of p-type Al_(0.15)Ga_(0.85)N having the thickness of about 20nm and the p-type light guiding layer 56 of p-type GaN having thethickness of about 100 nm are successively formed on the MQW activelayer.

Then, the p-type light spreading layer 6 of p-type Al_(0.15)Ga_(0.85)Nhaving the thickness of about 20 nm, the p-type cladding layer 7 ofAl_(0.03)Ga_(0.97)N having the thickness of about 0.3 μm and the p-typecontact layer 8 of p-type GaN having the thickness of about 0.07 μm aresuccessively formed on the emission layer 5 by MOCVD, as shown in FIG.4. In this crystal growth, Si and Mg are employed as n- and p-typedopants respectively.

Then, an SiO₂ film 15 having a thickness of about 0.2 μm is formed onthe overall surface of the p-type contact layer 8 by plasma CVD, asshown in FIG. 5. The SiO₂ film 15 is partially removed byphotolithography and hydrofluoric acid-based etching. RIE (reactive ionetching) is performed with chlorine-based gas up to intermediateportions of the p-type contact layer 8, the p-type cladding layer 7, thep-type light spreading layer 6, the emission layer 5, the n-type lightspreading layer 4, the n-type cladding layer 3 and the n-type contactlayer 2, thereby partially exposing the upper surface of the n-typecontact layer 2.

Then, the SiO₂ film 15 is patterned by photolithography and hydrofluoricacid-based etching, thereby forming a striped SiO₂ film 15 having awidth of about 2 μm, as shown in FIG. 6. The striped SiO₂ film 15 isemployed as an etching mask for partially removing the p-type contactlayer 8 and the p-type cladding layer 7 by RIE with chlorine-based gas,thereby forming the ridge portion 9. The depth of etching for formingthe ridge portion 9 is set to about 0.27 μm from the upper surface ofthe p-type contact layer 8. Thus, the portion of the p-type claddinglayer 7 excluding the ridge portion 9 has a thickness of about 0.1 μm.Thereafter the SiO₂ film 15 located on the ridge portion 9 is removed byhydrofluoric acid-based etching.

Then, the current blocking layer 10 of SiO₂ having the thickness ofabout 0.2 μm is formed by plasma CVD to cover the overall surface, asshown in FIG. 7. Parts of the current blocking layer 10 located on theridge portion 9 excluding a portion close to the cavity end surface anda part of the n-type contact layer 2 are removed by photolithography andRIE with CF₄ thereby partially exposing the upper surface of the p-typecontact layer 8 and the upper portion of the n-type contact layer 2, asshown in FIG. 2.

Finally, the p-side ohmic electrode 11 consisting of the lower Pt layerhaving the thickness of about 1 nm and the upper Pd layer having thethickness of about 3 nm is formed on the p-type contact layer 8 locatedon the ridge portion 9 excluding the portion close to the cavity endsurface by vapor deposition, as shown in FIG. 2. The p-side padelectrode 12 consisting of the lower Ni layer having the thickness ofabout 0.1 μm and the upper Au layer having the thickness of about 3 μmis formed on the p-side ohmic electrode 11 and the current blockinglayer 10. Further, the n-side ohmic electrode 13 consisting of the lowerTi layer having the thickness of about 10 nm and the upper Al layerhaving the thickness of about 0.1 μm is formed on the n-type contactlayer 2 by vapor deposition. In addition, the n-side pad electrode 14consisting of the lower Ni layer having the thickness of about 0.1 μmand the upper Au layer having the thickness of about 3 μm is formed onthe n-side ohmic electrode 13. Thus, the nitride-based semiconductorlaser device according to the first embodiment is fabricated.

Second Embodiment

Referring to FIG. 8, no light spreading layers are formed between anemission layer 5 and n- and p-type cladding layers 3 and 7 in anitride-based semiconductor laser device according to a secondembodiment of the present invention in a structure similar to thataccording to the aforementioned first embodiment. The remainingstructure of the nitride-based semiconductor laser device according tothe second embodiment and a fabrication method therefor aresubstantially similar to those of the first embodiment.

In the nitride-based semiconductor laser device according to the secondembodiment, n- and p-type carrier blocking layers 53 and 55 havinglarger band gaps than n- and p-type light guiding layers 54 and 56 areprovided between an MQW active layer and the n- and p-type light guidinglayers 54 and 56 respectively in the emission layer 5, similarly to thenitride-based semiconductor laser device according to the firstembodiment shown in FIG. 3. Also when light spreading is increased,therefore, the n- and p-type carrier blocking layers 53 and 55 caninhibit carriers from overflowing from the emission layer 5 into the n-and p-type cladding layers 3 and 7. Therefore, a threshold current andan operating current can be inhibited from increase resulting fromdifficulty in light emission followed by overflow of the carriers.Consequently, the device can be inhibited from deterioration resultingfrom rise of the internal temperature following increase of theoperating current, whereby the life of the device can be elongated.

According to the second embodiment, the Al compositions of the n- andp-type cladding layers 3 and 7 are so reduced as to reduce thedifference between the refractive indices of the emission layer 5 andthe n- and p-type cladding layers 3 and 7, whereby light spreading fromthe emission layer 5 to the n- and p-type cladding layers 3 and 7 can beincreased. The nitride-based semiconductor laser device according to thesecond embodiment provided with no light spreading layers has a smallerdegree of light spreading as compared with the nitride-basedsemiconductor laser device according to the first embodiment. In thenitride-based semiconductor laser device according to the secondembodiment, therefore, the vertical beam divergence angle in lasing isabout 17°, which is slightly larger than that (about 16°) in the firstembodiment. However, the vertical beam divergence angle can beremarkably reduced as compared with a conventional nitride-basedsemiconductor laser device prepared to have a large vertical beamdivergence angle of about 30°. Thus, optical density in the device canbe so reduced that light absorption resulting from crystal defects canbe reduced. Therefore, an operating current can be inhibited fromincrease resulting from light absorption. Also according to this, thedevice can be inhibited from deterioration resulting from rise of theinternal temperature following increase of the operating current,whereby the life of the device can be elongated.

Third Embodiment

Referring to FIG. 9, no carrier blocking layers are formed between anMQW active layer and n- and p-type light guiding layers 54 and 56 in anemission layer 5 in a nitride-based semiconductor laser device accordingto a third embodiment of the present invention in a structure similar tothat of the nitride-based semiconductor laser device according to thefirst embodiment. The remaining structure of and a fabrication methodfor the nitride-based semiconductor device according to the thirdembodiment are substantially similar to those of the nitride-basedsemiconductor laser device according to the first embodiment.

In the nitride-based semiconductor laser device according to the thirdembodiment, n- and p-type light spreading layers 4 and 6 having smallerrefractive indices and larger band gaps than n- and p-type claddinglayers 3 and 7 are provided between the emission layer 5 and the n- andp-type cladding layers 3 and 7 respectively similarly to thenitride-based semiconductor laser device according to the firstembodiment shown in FIG. 2. Thus, the n- and p-type light spreadinglayers 4 and 6 can increase light spreading from the emission layer 5 tothe n- and p-type cladding layers 3 and 7 due to the refractive indicessmaller than those of the n- and p-type cladding layers 3 and 7 andstrongly confine carriers in the emission layer 5 due to the band gapslarger than those of the n- and p-type cladding layers 3 and 7.Therefore, optical density in the device can be reduced. Thus, lightabsorption resulting from crystal defects can be so reduced that anoperating current can be inhibited from increase resulting from lightabsorption.

The carriers (electrons and holes), which can be strongly confined inthe emission layer 5 as hereinabove described, can be inhibited fromoverflowing from the emission layer 5 into the n- and p-type claddinglayers 3 and 7. Thus, a threshold current and the operating current canbe inhibited from increase followed by overflow of the carriers.

Thus, the operating current can be so inhibited from increase that thedevice can be inhibited from deterioration resulting from rise of theinternal temperature following increase of the operating current, andthe life of the device can consequently be elongated.

According to the third embodiment, each of the light spreading layers 4and 6 is separated from the MQW active layer by the thickness (about 100nm) of each of the light guiding layers 54 and 56. On the other hand,the carrier blocking layers 53 and 55 employed in the aforementionedsecond embodiment are in contact with the MQW active layer. The lightspreading layers 4 and 6 and the carrier blocking layers 53 and 55,which must have large band gaps, contain large quantities of Al (theselayers 4, 6, 53 and 55 consist of Al_(0.15)Ga_(0.85)N in the second andthird embodiments). Al, which is an active element, easily reacts withimpurity gas such as oxygen or carbon, for example, present in a crystalgrowth apparatus. Therefore, an impurity element such as oxygen orcarbon, for example, present in the crystal growth apparatus is easilyaccumulated on the interfaces between the light spreading layers 4 and 6and the carrier blocking layers 53 and 55 and the upper layers incontact with these layers 4, 6, 53 and 55, i.e., the interface betweenthe n-type light spreading layer 4 and the lower surface of the emissionlayer 5 (the lower surface of the n-type light guiding layer 54), theinterface between the p-type light spreading layer 6 and the lowersurface of the p-type cladding layer 7, the interface between the n-typecarrier blocking layer 53 and the lower surface of the lowermost quantumbarrier layer 52 and the interface between the p-type carrier blockinglayer 55 and the lower surface of the p-type light guiding layer 56. Theimpurity element may form an impurity level, to cause light absorptionthrough the impurity level. Consequently, the threshold current and theoperating current may be increased. According to the third embodiment,as hereinabove described, each of these interfaces can be separated fromthe MQW active layer by the thickness (about 100 nm) of each of thelight guiding layers 54 and 56 as compared with the second embodiment.Optical density is reduced as the interface is separated from the MQWactive layer, and hence light absorption caused on the aforementionedinterfaces is more reduced in the third embodiment as compared with thesecond embodiment. According to the third embodiment, therefore, thethreshold current and the operating current can be further inhibitedfrom increase as compared with the second embodiment.

According to the third embodiment, the difference between the refractiveindices of the emission layer 5 and the n- and p-type cladding layers 3and 7 is reduced and the n- and p-type light spreading layers 4 and 6are provided so that light spreading can be increased similarly to thefirst embodiment, whereby the vertical beam divergence angle in lasingcan be reduced to about 18°. In other words, the vertical beamdivergence angle can be remarkably reduced as compared with aconventional nitride-based semiconductor laser device prepared to have alarge vertical beam divergence angle of about 30°. According to thethird embodiment, the nitride-based semiconductor laser device has nocarrier blocking layers having a carrier blocking function and henceconfinement of carriers is weakened as compared with the firstembodiment.

While the light guiding layers 54 and 56 are provided on both sides ofthe MQW active layer, either light guiding layer 54 or 56 is morepreferably omitted. In this case, the MQW active layer is provided withonly a single light guiding layer for confining light, whereby opticalconfinement in the MQW active layer can be reduced as compared with thethird embodiment shown in FIG. 9. Therefore, light spreading to then-type cladding layer 3 or the p-type cladding layer 7 can be increased.The n- and p-type light spreading layers 4 and 6 having larger band gapsthan the n- and p-type cladding layers 3 and 7 are provided on both ofthe upper and lower surfaces of the emission layer 5, whereby carriers(electrons and holes) can be inhibited from overflowing from theemission layer 5 into the n- and p-type cladding layers 3 and 7.Consequently, the threshold current can be inhibited from increase whilethe vertical beam divergence angle in lasing can be reduced to about 16°similarly to the first embodiment when either light guiding layer 54 or56 is omitted in the third embodiment.

Fourth Embodiment

Referring to FIG. 10, a conductive n-type GaN substrate 61 is employedin a nitride-based semiconductor laser device according to a fourthembodiment of the present invention in place of the sapphire substrate 1employed in the first embodiment.

In the nitride-based semiconductor laser device according to the fourthembodiment, an n-side ohmic electrode 13 and an n-side pad electrode 14are formed on the back surface of the conductive n-type GaN substrate61, dissimilarly to the nitride-based semiconductor laser deviceaccording to the first embodiment. Further, regions from a p-typecontact layer 8 to an n-type contact layer 2 are not partially removedalso dissimilarly to the nitride-based semiconductor laser deviceaccording to the first embodiment. The remaining structure of thenitride-based semiconductor laser device according to the fourthembodiment is substantially similar to that of the nitride-basedsemiconductor laser device according to the first embodiment.

Effects of the nitride-based semiconductor laser device according to thefourth embodiment are similar to those of the aforementioned firstembodiment. In other words, n- and p-type carrier blocking layers 53 and55 having larger band gaps than n- and p-type light guiding layers 54and 56 can inhibit carriers from overflowing from an emission layer 5into n- and p-type cladding layers 3 and 7 also when light spreading isincreased, whereby a threshold current and an operating current can beinhibited from increase.

Further, n- and p-type light spreading layers 4 and 6 can increase lightspreading from the emission layer 5 to the n- and p-type cladding layers3 and 7 due to refractive indices smaller than those of the n- andp-type cladding layers 3 and 7 and strongly confine carriers in theemission layer 5 due to band gaps larger than those of the n- and p-typecladding layers 3 and 7. Thus, light spreading can be so increased as toreduce light absorption resulting from crystal defects. Therefore, theoperating current can be inhibited from increase resulting from lightabsorption. The operating current can be inhibited from increase asdescribed above, whereby the device can be inhibited from deteriorationresulting from rise of the internal temperature following increase ofthe operating current. Consequently, the life of the device can beelongated.

The nitride-based semiconductor laser device according to the fourthembodiment can increase light spreading as hereinabove described,whereby the vertical beam divergence angle in lasing can be reduced toabout 16°, similarly to the first embodiment.

A method of fabricating the nitride-based semiconductor laser deviceaccording to the fourth embodiment is now described with reference toFIGS. 11 to 14.

First, the n-type contact layer 2 of n-type GaN having a thickness ofabout 4 μm, the n-type cladding layer 3 of n-type Al_(0.03)Ga_(0.97)Nhaving a thickness of about 1 μm, the n-type light spreading layer 4 ofn-type Al_(0.15)Ga_(0.85)N having a thickness of about 20 nm and theemission layer 5 consisting of a multilevel film structure aresuccessively formed on the n-type GaN substrate 61 by MOCVD, as shown inFIG. 11.

In order to form the emission layer 5 consisting of a multilevel filmstructure, the n-type light guiding layer 54 of n-type GaN having athickness of about 100 nm, the n-type carrier blocking layer 53 ofn-type Al_(0.15)Ga_(0.85)N having a thickness of about 20 nm, an MQWactive layer, prepared by alternately stacking four quantum barrierlayers 52 of In_(y)Ga_(1-y)N each having a thickness of about 20 nm andthree quantum well layers 51 of In_(x)Ga_(1-x)N each having a thicknessof about 4 nm, the p-type carrier blocking layer 55 of p-typeAl_(0.15)Ga_(0.85)N having a thickness of about nm and the p-type lightguiding layer 56 of p-type GaN having a thickness of about 100 nm aresuccessively formed on the n-type light spreading layer 4 through aprocess similar to that for the nitride-based semiconductor laser deviceaccording to the first embodiment shown in FIG. 3.

Then, the p-type light spreading layer 6 of p-type Al_(0.15)Ga_(0.85)Nhaving a thickness of about 20 nm, the p-type cladding layer 7 ofAl_(0.03)Ga_(0.97)N having a thickness of about 0.3 μm and the p-typecontact layer 8 of p-type GaN having a thickness of about 0.07 μm aresuccessively formed on the emission layer 5 by MOCVD. In theaforementioned crystal growth, Si and Mg are employed as n- and p-typedopants respectively.

Then, an SiO₂ film 15 having a thickness of about 0.2 μm is formed onthe overall surface of the p-type contact layer 8 by plasma CVD, asshown in FIG. 12. The SiO₂ film 15 is patterned by photolithography andhydrofluoric acid-based etching, thereby forming a striped SiO₂ film 15having a width of about 2 μm as shown in FIG. 13. The striped SiO₂ film15 is employed as an etching mask for partially removing the p-typecontact layer 8 and the p-type cladding layer 7 by RIE withchlorine-based gas, thereby forming a ridge portion 9. The depth ofetching for forming the ridge portion 9 is set to about 0.27 μm from theupper surface of the p-type contact layer 8. Thus, the portion of thep-type cladding layer 7 excluding the ridge portion 9 has a thickness ofabout 0.1 μm. Thereafter the SiO₂ film 15 located on the ridge portion 9is removed by hydrofluoric acid-based etching.

Then, a current blocking layer 10 of SiO₂ having a thickness of about0.2 μm is formed by plasma CVD to cover the overall surface, as shown inFIG. 14. A part of the current blocking layer 10 located on the ridgeportion 9 excluding a portion close to the cavity end surface is removedby photolithography and RIE with CF₄ thereby partially exposing theupper surface of the p-type contact layer 8, as shown in FIG. 10.

Finally, a p-side ohmic electrode 11 consisting of a lower Pt layerhaving a thickness of about 1 nm and an upper Pd layer having athickness of about 3 nm is formed on the p-type contact layer 8 locatedon the ridge portion 9 excluding the portion close to the cavity endsurface by vapor deposition, as shown in FIG. 10. A p-side pad electrode12 consisting of a lower Ni layer having a thickness of about 0.1 μm andan upper Au layer having a thickness of about 3 μm is formed on thep-side ohmic electrode 11 and the current blocking layer 10. The backsurface of the n-type GaN substrate 61 is polished by lapping until thesubstrate 61 exhibits a thickness of about 100 μm. Thereafter an n-sideohmic electrode 13 consisting of a lower Ti layer having a thickness ofabout 10 nm and an upper Al layer having a thickness of about 0.1 μm isformed on the back surface of the n-type GaN substrate 61. In addition,an n-side pad electrode 14 consisting of a lower Ni layer having athickness of about 0.1 μm and an upper Au layer having a thickness ofabout 3 μm is formed under the n-side ohmic electrode 13. Thus, thenitride-based semiconductor laser device according to the fourthembodiment is fabricated.

Fifth Embodiment

Referring to FIGS. 15 and 16, quaternary mixed crystal materials ofAlInGaN are employed in a nitride-based semiconductor laser deviceaccording to a fifth embodiment of the present invention in place of theternary mixed crystal materials of AlGaN employed in each of thenitride-based semiconductor laser devices according to theaforementioned first to fourth embodiments.

More specifically, n- and p-type cladding layers 73 and 77 consisting ofAl_(0.12)In_(0.05)Ga_(0.83)N are employed in place of the n- and p-typecladding layers 3 and 7 consisting of Al_(0.03)Ga_(0.97)N employed inthe first to fourth embodiments. Further, n- and p-type light spreadinglayers 74 and 76 and n- and p-type carrier blocking layers 753 and 755consisting of Al_(0.24)In_(0.05)Ga_(0.71)N are employed in place of then- and p-type light spreading layers 4 and 6 and the n- and p-typecarrier blocking layers 53 and 55 consisting of Al_(0.15)Ga_(0.85)Nemployed in the first to fourth embodiments.

The n- and p-type light spreading layers 74 and 76 have smallerrefractive indices and larger band gaps than the n- and p-type claddinglayers 73 and 77. Further, the n- and p-type carrier blocking layers 753and 755 have larger band gaps than n- and p-type light guiding layers 54and 56. The n- and p-type carrier blocking layers 753 and 755 havefunctions of inhibiting carriers from overflowing and spreading light.The n-type cladding layer 73 is an example of the “first cladding layer”in the present invention, and the p-type cladding layer 77 is an exampleof the “second cladding layer” in the present invention. The n- andp-type light spreading layers 74 and 76 are examples of the “lightspreading layer” in the present invention. The n- and p-type carrierblocking layers 753 and 755 are examples of the “carrier blocking layer”in the present invention. The remaining structure of and a fabricationmethod for the nitride-based semiconductor laser device according to thefifth embodiment are substantially similar to those in the firstembodiment.

The quaternary mixed crystal material Al_(0.12)In_(0.05)Ga_(0.83)Nemployed for the n- and p-type cladding layers 73 and 77 in the fifthembodiment has a refractive index of 2.540, which is substantiallyidentical to that of the ternary mixed crystal materialAl_(0.03)Ga_(0.97)N employed in the first to fourth embodiments.Further, the quaternary mixed crystal materialAl_(0.24)In_(0.05)Ga_(0.71)N employed for the n- and p-type lightspreading layers 74 and 76 and the n- and p-type carrier blocking layers753 and 755 in the fifth embodiment has a refractive index of 2.501,which is substantially identical to that of the ternary mixed crystalmaterial Al_(0.15)Ga_(0.85)N employed in the first to fourthembodiments. Thus, the nitride-based semiconductor laser deviceaccording to the fifth embodiment can attain effects similar to those ofthe first to fourth embodiments in optical confinement.

Further, the quaternary mixed crystal materialAl_(0.12)In_(0.05)Ga_(0.83)N employed for the n- and p-type claddinglayers 73 and 77 in the fifth embodiment has a larger band gap than theternary mixed crystal material Al_(0.03)Ga_(0.97)N, while the quaternarymixed crystal material Al_(0.24)In_(0.05)Ga_(0.71)N employed for the n-and p-type light spreading layers 74 and 76 and the n- and p-typecarrier blocking layers 753 and 755 in the fifth embodiment also has alarger band gap than the ternary mixed crystal materialAl_(0.15)Ga_(0.85)N. Comparing ternary and quaternary mixed crystalmaterials having the same refractive indices with each other, therefore,the quaternary mixed crystal material has a larger band gap than theternary mixed crystal material.

FIG. 17 is a graph showing the relation between the band gaps and therefractive indices of ternary and quaternary mixed crystal materials inrelation to the fifth embodiment of the present invention obtained bycalculation. Referring to FIG. 17, a straight line of AlInGaN (In=2%)shows the relation between the band gaps and the refractive indices witha constant In composition of 2% and various Al and Ga compositions. Thegeneral formula of the composition of AlInGaN (In=2%) isAl_(w)In_(0.02)Ga_((0.98-w))N. A straight line of AlInGaN (In=5%) showsthe relation between the band gaps and the refractive indices with aconstant In composition of 5% and various Al and Ga compositions. Thegeneral formula of the composition of AlInGaN (In=5%) isAl_(v)In_(0.05)Ga_((0.95-v))N.

Referring to FIG. 17, the band gaps and the refractive indices are insubstantially linear relation while the band gaps are increased as therefractive indices are reduced in the straight lines of all ternary andquaternary mixed crystal materials. Further, the straight line of thequaternary mixed crystal material AlInGaN (In=2%) is positioned abovethe straight line of the ternary mixed crystal material AlGaN, while thestraight line of the quaternary mixed crystal material AlInGaN (In=5%)is positioned above the straight line of the quaternary mixed crystalmaterial AlInGaN (In=2%). Thus, the band gaps are increased in order ofAlGaN, Al_(w)In_(0.02)Ga_((0.98-w))N and Al_(v)In_(0.05)Ga_((0.95-v))Nat the same refractive index. At the refractive index of 2.52, forexample, AlGaN, Al_(w)In_(0.02)Ga_((0.98-w))N andAl_(v)In_(0.05)Ga_((0.95-v))N exhibit band gaps of 3.61, 3.69 and 3.79respectively. Thus, the band gaps are increased as the quantity of In isincreased for the following reason:

The refractive index of each material is increased as the quantity of Inis increased. In order to keep the refractive index exhibited beforeaddition of In, it is necessary to reduce the refractive index in thestate where In is added. Thus, the Al composition must be increased.When the Al composition is increased, the band gap of AlInGaN is alsoincreased. Thus, the band gap is increased as the quantity of In isincreased.

According to the fifth embodiment, the quaternary mixed crystalmaterials of AlInGaN having larger band gaps than the ternary mixedcrystal materials of AlGaN employed in the first to fourth embodimentsare so employed that carriers can be more inhibited from overflowing ascompared with the nitride-based semiconductor laser devices according tothe first to fourth embodiments, whereby a threshold current and anoperating current can be remarkably inhibited from increase resultingfrom difficulty in light emission followed by overflow of the carriers.Consequently, the device can be further inhibited from deteriorationresulting from rise of the internal temperature following increase ofthe operating current, whereby the life of the device can be furtherelongated as compared with the first embodiment.

The remaining effects of the nitride-based semiconductor laser deviceaccording to the fifth embodiment are similar to those of theaforementioned first embodiment. In other words, the n- and p-typecarrier blocking layers 753 and 755 having larger band gaps than the n-and p-type light guiding layers 54 and 56 can inhibit carriers fromoverflowing from an emission layer 5 into the n- and p-type claddinglayers 73 and 77 also when light spreading is increased, whereby thethreshold current and the operating current can be inhibited fromincrease.

Further, the n- and p-type light spreading layers 74 and 76 can increaselight spreading from the emission layer 5 into the n- and p-typecladding layers 73 and 77 due to the refractive indices smaller thanthose of the n- and p-type cladding layers 73 and 77 and stronglyconfine carriers in the emission layer 5 due to the band gaps largerthan those of the n- and p-type cladding layers 73 and 77. Thus, lightspreading can be so increased as to reduce light absorption resultingfrom crystal defects. Therefore, the operating current can be inhibitedfrom increase resulting from light absorption. Thus, the operatingcurrent can be so inhibited from increase that the device can beinhibited from deterioration resulting from rise of the internaltemperature following increase of the operating current. Consequently,the life of the device can be elongated.

In the nitride-based semiconductor laser device according to the fifthembodiment, light spreading can be increased as hereinabove described,whereby the vertical beam divergence angle in lasing can be reduced toabout 16°, similarly to the first embodiment.

Sixth Embodiment

In a nitride-based semiconductor laser device according to a sixthembodiment of the present invention shown in FIGS. 18 and 19, no n- andp-type light spreading layers consisting of Al_(0.24)In_(0.05)Ga_(0.71)Nare formed but only a p-type carrier blocking layer 755 consisting of aquaternary mixed crystal material Al_(0.24)In_(0.05)Ga_(0.71)N isprovided in a structure similar to that of the aforementioned fifthembodiment. The remaining structure of and a fabrication method for thenitride-based semiconductor laser device according to the sixthembodiment are substantially similar to those in the fifth embodiment.

In the nitride-based semiconductor laser device according to the sixthembodiment, the p-type carrier blocking layer 755 having a larger bandgap than a p-type light guiding layer 56 is provided between an MQWactive layer and the p-type light guiding layer 56 in an emission layer5, as shown in FIG. 19. Thus, the p-type carrier blocking layer 755 caninhibit carriers from overflowing from the emission layer 5 into ap-type cladding layer 77 also when the difference between the refractiveindices of the emission layer 5 and the p-type cladding layer 77 isreduced thereby increasing light spreading. Thus, a threshold currentand an operating current can be inhibited from increase resulting fromdifficulty in light emission followed by overflow of the carriers.

According to the aforementioned sixth embodiment, the difference betweenthe refractive indices of n- and p-type cladding layers 73 and 77 andthe emission layer 5 can be reduced by increasing Al compositions of then- and p-type cladding layers 73 and 77, whereby light spreading can beincreased. In the nitride-based semiconductor laser device according tothe sixth embodiment provided with neither n and p-type light spreadinglayers nor n-type carrier blocking layer, light spreading is reduced ascompared with the fifth embodiment. Therefore, the vertical beamdivergence angle in lasing is slightly increased to about 18° in thesixth embodiment as compared with that (about 16°) in the fifthembodiment. As compared with a conventional nitride-based semiconductorlaser device prepared to have a large vertical beam divergence angle ofabout 30°, however, the vertical beam divergence angle can be remarkablyreduced.

According to the sixth embodiment, further, a quaternary mixed crystalmaterial of AlInGaN having a larger band gap than the ternary mixedcrystal materials of AlGaN employed in the first to fourth embodimentsso that the carriers can be further inhibited from overflowing similarlyto the fifth embodiment, whereby the threshold current and the operatingcurrent can be remarkably inhibited from increase. Consequently, thedevice can be further inhibited from deterioration resulting from riseof the internal temperature following increase of the operating current,whereby the life of the device can be remarkably elongated.

Seventh Embodiment

Referring to FIGS. 20 and 21, ternary mixed crystal materials of AlInGaNlattice-matching with GaN are employed in place of the ternary mixedcrystal materials of AlGaN employed in the aforementioned first tofourth embodiments in a nitride-based semiconductor laser deviceaccording to a seventh embodiment of the present invention.

More specifically, n- and p-type cladding layers 83 and 87 consisting ofAl_(0.04)In_(0.005)Ga_(0.955)N are employed in place of the n- andp-type cladding layers 3 and 7 consisting of Al_(0.03)Ga_(0.97)Nemployed in the first to fourth embodiments. Further, n- and p-typelight spreading layers 84 and 86 and n- and p-type carrier blockinglayers 853 and 855 consisting of Al_(0.19)In_(0.025)Ga_(0.785)N areemployed in place of the n- and p-type light spreading layers 4 and 6and the n- and p-type carrier blocking layers 53 and 55 consisting ofAl_(0.15)Ga_(0.85)N employed in the first to fourth embodiments. Inaddition, the aforementioned quaternary mixed crystal materials employedin the seventh embodiment, having lattice constants substantiallyidentical to that of GaN, lattice-match with GaN dissimilarly to thequaternary mixed crystal materials employed in the fifth and sixthembodiments.

The n- and p-type light spreading layers 84 and 86 have smallerrefractive indices and larger band gaps than the n- and p-type claddinglayers 83 and 87. Further, the n- and p-type carrier blocking layers 853and 855 have larger band gaps than n- and p-type light guiding layers 54and 56. The n- and p-type carrier blocking layers 853 and 855 havefunctions of inhibiting carriers from overflowing and spreading light.The n-type cladding layer 83 is an example of the “first cladding layer”in the present invention, and the p-type cladding layer 87 is an exampleof the “second cladding layer” in the present invention. The n- andp-type light spreading layers 84 and 86 are examples of the “lightspreading layer” in the present invention. The n- and p-type carrierblocking layers 853 and 855 are examples of the “carrier blocking layer”in the present invention. The remaining structure of and a fabricationmethod for the seventh embodiment are similar to those in the firstembodiment.

FIG. 22 is a graph showing the relation between band gaps and refractiveindices of the quaternary mixed crystal material lattice-matching withGaN employed in the seventh embodiment of the present invention and aternary mixed crystal material. Referring to FIG. 22, a straight lineshowing the quaternary mixed crystal material lattice-matching with GaNis positioned above a straight line showing the ternary mixed crystalmaterial AlGaN. Thus, the band gap of the quaternary mixed crystalmaterial lattice-matching with GaN is larger than that of the ternarymixed crystal material at the same refractive index.

The quaternary mixed crystal material Al_(0.04)In_(0.005)Ga_(0.955)Nemployed for the n- and p-type cladding layers 83 and 87 in the seventhembodiment has a refractive index of 2.540, which is substantiallyidentical to that of the ternary mixed crystal materialAl_(0.03)Ga_(0.97)N employed in the first to fourth embodiments. Thequaternary mixed crystal material Al_(0.19)In_(0.025)Ga_(0.785)Nemployed for the n- and p-type light spreading layers 84 and 86 and then- and p-type carrier blocking layers 853 and 855 in the seventhembodiment has a refractive index of 2.501, which is substantiallyidentical to that of the ternary mixed crystal materialAl_(0.15)Ga_(0.85)N employed in the first to fourth embodiments. Thus,the nitride-based semiconductor laser device according to the seventhembodiment can attain effects similar to those in the first to fourthembodiments in optical confinement. The quaternary mixed crystalmaterials Al_(0.04)In_(0.005)Ga_(0.955)N andAl_(0.19)In_(0.025)Ga_(0.785)N lattice-matching with GaN have largerband gaps than the ternary mixed crystal materials Al_(0.03)Ga_(0.97)Nand Al_(0.15)Ga_(0.85)N respectively.

According to the seventh embodiment, the quaternary mixed crystalmaterials of AlInGaN having larger band gaps than the ternary mixedcrystal materials of AlGaN employed in the first to fourth embodimentsso that carriers can be further inhibited from overflowing as comparedwith the nitride-based semiconductor laser devices according to thefirst to fourth embodiments, whereby a threshold current and anoperating current can be remarkably inhibited from increase resultingfrom difficulty in light emission followed by overflow of the carriers.Consequently, the device can be further inhibited from deteriorationresulting from rise of the internal temperature following increase ofthe operating current, whereby the life of the device can be furtherelongated as compared with the first embodiment.

According to the seventh embodiment, the quaternary mixed crystalshaving the same lattice constant as that of the material for an n-typeGaN contact layer 2 are employed in place of the quaternary mixedcrystal materials of AlInGaN employed in the fifth and sixthembodiments, whereby formation of crystal defects resulting fromdifference between lattice constants can be remarkably suppressed ascompared with the nitride-based semiconductor laser devices according tothe fifth and sixth embodiments. Thus, high-quality crystals can beobtained and light absorption resulting from crystal defects can bereduced, whereby the operating current can be remarkably inhibited fromincrease resulting from light absorption. Consequently, the device canbe further inhibited from deterioration resulting from rise of theinternal temperature following increase of the operating current,whereby the life of the device can be remarkably elongated.

The remaining effects of the nitride-based semiconductor laser deviceaccording to the seventh embodiment are similar to those of theaforementioned first embodiment. In other words, the n- and p-typecarrier blocking layers 853 and 855 having larger band gaps than the n-and p-type light guiding layers 54 and 56 can inhibit carriers fromoverflowing from an emission layer 5 into the n- and p-type claddinglayers 83 and 87 also when light spreading is increased, whereby thethreshold current and the operating current can be inhibited fromincrease.

Further, the n- and p-type light spreading layers 84 and 86 can increaselight spreading from the emission layer 5 to the n- and p-type claddinglayers 83 and 87 due to the refractive indices smaller than those of then- and p-type cladding layers 83 and 87 and strongly confine carriers inthe emission layer 5 due to the band gaps larger than those of the n-and p-type cladding layers 83 and 87. Thus, light spreading can be soincreased that light absorption resulting from crystal defects can bereduced. Therefore, the operating current can be inhibited from increaseresulting from light absorption. The operating current can be inhibitedfrom increase as hereinabove described, whereby the device can beinhibited from deterioration resulting from rise of the internaltemperature following increase of the operating current. Consequently,the life of the device can be elongated.

In the nitride-based semiconductor laser device according to the seventhembodiment, further, light spreading can be increased as hereinabovedescribed, whereby the vertical beam divergence angle in lasing can bereduced to about 16° similarly to the first embodiment.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

For example, while no buffer layer is formed between the substrate andthe n-type contact layer in the nitride-based semiconductor laser deviceaccording to each of the aforementioned first to seventh embodiments,the present invention is not restricted to this but a low-temperaturebuffer layer consisting of AlN, GaN or AlGaN may alternatively be formedbetween the substrate and the n-type contact layer. Further, ahigh-temperature buffer layer consisting of undoped AlN, GaN or AlGaNmay be formed on the low-temperature buffer layer.

While an n-type cladding layer 3 having a thickness of about 1 μm isformed in the aforementioned seventh embodiment, the present inventionis not restricted to this but the n-type cladding layer 3 mayalternatively have a large thickness of at least 1.5 μm due to theemployment of the quaternary mixed crystal materials having latticeconstants equivalent to that of the n-type contact layer 2. In thiscase, light spreading to the n-type cladding layer 3 can be furtherincreased, whereby the vertical beam divergence angle can be furtherreduced.

While the n- and p-type light spreading layers 84 and 86 and the n- andp-type carrier blocking layers 853 and 855 consisting ofAl_(0.19)In_(0.025)Ga_(0.785)N are formed in the aforementioned seventhembodiment, the present invention is not restricted to this but theselayers 84, 86, 853 and 855 may not be formed. In this case, an effectsimilar to that of the sixth embodiment can be attained. In addition tothis effect, further, formation of crystal defects resulting fromdifference between lattice constants can be suppressed due to thequaternary mixed crystal material having the lattice constant equivalentto that of the material for the n-type GaN contact layer 2. Thus,high-quality crystals can be obtained while light absorption resultingfrom crystal defects can be reduced, whereby the operating current canbe further inhibited from increase resulting from light absorption.Further, the n-type cladding layer 3 can be formed with a largethickness of at least 1.5 μm due to the quaternary mixed crystalmaterial having the lattice constant equivalent to that of the materialfor the n-type contact layer 2. In this case, light spreading to then-type cladding layer 3 can be further increased, whereby the verticalbeam divergence angle can be further reduced.

1-23. (canceled)
 24. A nitride-based semiconductor laser devicecomprising: a first cladding layer consisting of a first conductivitytype nitride-based semiconductor; an emission layer, formed on saidfirst cladding layer, consisting of a nitride-based semiconductor; and asecond cladding layer, formed on said emission layer, consisting of asecond conductivity type nitride-based semiconductor, said nitride-basedsemiconductor laser device reducing the degree of optical confinement insaid emission layer thereby reducing a vertical beam divergence angle tonot more than 20°.